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Tuesday, 28 May 2013

Down syndrome, the most common genetic form of intellectual disability, results from an extra copy of one chromosome. Although people with Down syndrome experience intellectual difficulties and other problems, scientists have had trouble identifying why that extra chromosome causes such widespread effects.

In new research published this week, Anita Bhattacharyya, a neuroscientist at the Waisman Center at the University of Wisconsin-Madison, reports on brain cells that were grown from skin cells of individuals with Down syndrome.

"Even though Down syndrome is very common, it's surprising how little we know about what goes wrong in the brain," says Bhattacharyya. "These new cells provide a way to look at early brain development."

The study began when those skin cells were transformed into induced pluripotent stem cells, which can be grown into any type of specialized cell. Bhattacharyya's lab, working with Su-Chun Zhang and Jason Weick, then grew those stem cells into brain cells that could be studied in the lab.

One significant finding was a reduction in connections among the neurons, Bhattacharyya says.

"They communicate less, are quieter. This is new, but it fits with what little we know about the Down syndrome brain."

Brain cells communicate through connections called synapses, and the Down neurons had only about 60 percent of the usual number of synapses and synaptic activity.

"This is enough to make a difference," says Bhattacharyya.

"Even if they recovered these synapses later on, you have missed this critical window of time during early development."

The researchers looked at genes that were affected in the Down syndrome stem cells and neurons, and found that genes on the extra chromosome were increased 150 percent, consistent with the contribution of the extra chromosome.

However, the output of about 1,500 genes elsewhere in the genome was strongly affected.

"It's not surprising to see changes, but the genes that changed were surprising," says Bhattacharyya. The predominant increase was seen in genes that respond to oxidative stress, which occurs when molecular fragments called free radicals damage a wide variety of tissues.

"We definitely found a high level of oxidative stress in the Down syndrome neurons," says Bhattacharyya.

"This has been suggested before from other studies, but we were pleased to find more evidence for that. We now have a system we can manipulate to study the effects of oxidative stress and possibly prevent them."

Down syndrome includes a range of symptoms that could result from oxidative stress, Bhattacharyya says, including accelerated aging.

"In their 40s, Down syndrome individuals age very quickly. They suddenly get gray hair; their skin wrinkles, there is rapid aging in many organs, and a quick appearance of Alzheimer's disease. Many of these processes may be due to increased oxidative stress, but it remains to be directly tested."

Oxidative stress could be especially significant, because it appears right from the start in the stem cells.

"This suggests that these cells go through their whole life with oxidative stress," Bhattacharyya adds, "and that might contribute to the death of neurons later on, or increase susceptibility to Alzheimer's."

Other researchers have created neurons with Down syndrome from induced pluripotent stem cells, Bhattacharyya notes.

"However, we are the first to report this synaptic deficit, and to report the effects on genes on other chromosomes in neurons. We are also the first to use stem cells from the same person that either had or lacked the extra chromosome. This allowed us to look at the difference just caused by extra chromosome, not due to the genetic difference among people."

The research, published the week of May 27 in the Proceedings of the National Academy of Sciences, was a basic exploration of the roots of Down syndrome. Bhattacharyya says that while she did not intend to explore treatments in the short term, "we could potentially use these cells to test or intelligently design drugs to target symptoms of Down syndrome."

Human foetal stem cell grafts improve both motor and sensory functions in rats suffering from a spinal cord injury, according to research published this week in BioMed Central's open access journal Stem Cell Research and Therapy. This cell replacement therapy also improves the structural integrity of the spine, providing a functional relay through the injury site. The research gives hope for the treatment of spinal cord injuries in humans.

Grafting human neural stem cells into the spine is a promising approach to promote the recovery of function after spinal injury. Sebastian van Gorp, from the University of California San Diego, and team's work looks specifically at the effect of intraspinal grafting of human foetal spinal cord-derived neural stem cells on the recovery of neurological function in a rats with acute lumbar compression injuries.

A total of 42 three month-old female Sprague-Dawley rats, with spinal compression injuries, were allocated to one of three groups. The rats in the first group received a spinal injection with the stem cells, those in the second group received a placebo injection, while those in the third group received no injection.

Treatment effectiveness was assessed by a combination of measures, including motor and sensory function tests, presence of muscle spasticity and rigidity which causes stiffness and limits residual movement. The team also evaluated of how well the grafted cells had integrated into the rodents' spines.

Gorp and colleagues found that, compared to rats who received either the placebo injection or no injection, those who received the stem cell grafts showed a progressive and significant improvement in gait/paw placement, reduced muscle spasticity as well as improved sensitivity to both mechanical and thermal stimuli. In addition to these behavioural benefits, the researchers observed long-term improvements in the structural integrity of previously injured spinal cord segments.

Thursday, 16 May 2013

The breakthrough marks the first time human stem cells have been produced via nuclear transfer and follows several unsuccessful attempts by research groups worldwide

Thursday, 16 May 2013

Scientists at Oregon Health & Science University and the Oregon National Primate Research Center (ONPRC) have successfully reprogrammed human skin cells to become embryonic stem cells capable of transforming into any other cell type in the body. It is believed that stem cell therapies hold the promise of replacing cells damaged through injury or illness. Diseases or conditions that might be treated through stem cell therapy include Parkinson's disease, multiple sclerosis, cardiac disease and spinal cord injuries.

The research breakthrough, led by Shoukhrat Mitalipov, Ph.D., a senior scientist at ONPRC, follows previous success in transforming monkey skin cells into embryonic stem cells in 2007. This latest research will be published in the journal Cell online May 15 and in print June 6.

Scematic
description of how to create

stem cells from human
skin. Credit:

ONPRC.

The technique used by Drs. Mitalipov, Paula Amato, M.D., and their colleagues in OHSU's Division of Reproductive Endocrinology and Infertility, Department of Obstetrics & Gynecology, is a variation of a commonly used method called somatic cell nuclear transfer, or SCNT. It involves transplanting the nucleus of one cell, containing an individual's DNA, into an egg cell that has had its genetic material removed. The unfertilized egg cell then develops and eventually produces stem cells.

"Such stem cells can regenerate and replace those damaged cells and tissues and alleviate diseases that affect millions of people."

"A thorough examination of the stem cells derived through this technique demonstrated their ability to convert just like normal embryonic stem cells, into several different cell types, including nerve cells, liver cells and heart cells. Furthermore, because these reprogrammed cells can be generated with nuclear genetic material from a patient, there is no concern of transplant rejection," explained Dr. Mitalipov.

"While there is much work to be done in developing safe and effective stem cell treatments, we believe this is a significant step forward in developing the cells that could be used in regenerative medicine."

Another noteworthy aspect of this research is that it does not involve the use of fertilized embryos, a topic that has been the source of a significant ethical debate.

The
first step during SCNT is enucleation

or
removal of nuclear genetic material

(chromosomal)
from a human egg. An

egg is
positioned with holding pipette

(on the
left) and egg's chromosomes are

visualized
under polarized microscope.

A hole
is made in the egg's shell (zone

pellucida)
using a laser and a smaller

pipette
(on the right) is inserted through

the
opening. The chromosomes then

sucked
in inside the pipette and slowly

removed
from the egg. Credit: Cell,

Tachibana
et al..

The Mitalipov team's success in reprogramming human skin cells came through a series of studies in both human and monkey cells. In the past, researchers have used SCNT to generate only mouse and monkey embryonic stem cells — immature cells that can develop into different types of specialized cells, from neurons to heart muscle cells. Most previous attempts failed to produce human SCNT embryos that could progress beyond the 8-cell stage, falling far short of the 150-cell blastocyst stage that could provide hESCs for clinical purposes. Until now, it was not clear which factors and protocols are important for promoting SCNT embryonic development. Previous unsuccessful attempts by several labs showed that human egg cells appear to be more fragile than eggs from other species. Therefore, known reprogramming methods stalled before stem cells were produced.

To solve this problem, the OHSU group studied various alternative approaches first developed in monkey cells and then applied to human cells. Through moving findings between monkey cells and human cells, the researchers were able to develop a successful method.

The key to this success was finding a way to prompt egg cells to stay in a state called "metaphase" during the nuclear transfer process. Metaphase is a stage in the cell's natural division process (meiosis) when genetic material aligns in the middle of the cell before the cell divides. The research team found that chemically maintaining metaphase throughout the transfer process prevented the process from stalling and allowed the cells to develop and produce stem cells.

This is
a colony of human ESCs (upper

portion)
extracted from a blastocyst

generated
by SCNT Credit: Cell,

Tachibana
et al..

"This is a remarkable accomplishment by the Mitalipov lab that will fuel the development of stem cell therapies to combat several diseases and conditions for which there are currently no treatments or cures," said Dr. Dan Dorsa, Ph.D., OHSU Vice President for Research.

"The achievement also highlights OHSU's deep reproductive expertise across our campuses. A key component to this success was the translation of basic science findings at the OHSU primate centre paired with privately funded human cell studies."

One important distinction is that while the method might be considered a technique for cloning stem cells, commonly called therapeutic cloning, the same method would not likely be successful in producing human clones otherwise known as reproductive cloning. Several years of monkey studies that utilize somatic cell nuclear transfer have never successfully produced monkey clones. It is expected that this is also the case with humans. Furthermore, the comparative fragility of human cells as noted during this study is a significant factor that would likely prevent the development of clones.

"Our research is directed toward generating stem cells for use in future treatments to combat disease," added Dr. Mitalipov.

"While nuclear transfer breakthroughs often lead to a public discussion about the ethics of human cloning, this is not our focus, nor do we believe our findings might be used by others to advance the possibility of human reproductive cloning."

Thursday, 9 May 2013

Collaborative study will help overcome hurdles to using stem cells to treat diseases and injuries

Thursday, 09 May 2013

Scientists have long known that control mechanisms known collectively as "epigenetics" play a critical role in human development, but they did not know precisely how alterations in this extra layer of biochemical instructions in DNA contribute to development.

Now, in the first comprehensive analysis of epigenetic changes that occur during development, a multi-institutional group of scientists, including several from the Salk Institute for Biological Studies, has discovered how modifications in key epigenetic markers influence human embryonic stem cells as they differentiate into specialized cells in the body. The findings were published May 9 in Cell.

Professor Joseph R. Ecker, Plant
Molecular and

Cellular Biology Laboratory, Howard
Hughes

Medical Institute and Gordon and Betty
Moore

Foundation Investigator, Salk
International

Council Chair in Genetics. Credit:
Courtesy of

the Salk Institute for Biological Studies.

"Our findings help us to understand processes that occur during early human development and the differentiation of a stem cell into specialized cells, which ultimately form tissues in the body," says co-lead author Joseph R. Ecker, a professor and director of Salk's Plant Molecular and Cellular Biology Laboratory and holder of the Salk International Council Chair in Genetics.

Scientists have established that the gene expression program encoded in DNA is carried out by proteins that bind to regulatory genes and modulate gene expression in response to environmental cues. Growing evidence now shows that maintenance of this process depends on epigenetic marks such as DNA methylation and chromatin modifications, biochemical processes that alter gene expression as cells divide and differentiate from embryonic stem cells into specific tissues. Epigenetic modifications - collectively known as the epigenome - control which genes are turned on or off without changing the letters of the DNA alphabet (A-T-C-G), providing cells with an additional tool to fine-tune how genes control the cellular machinery.

In their study, the Salk researchers and their collaborators from several prominent research institutions across the United States examined the beginning state of cells, before and after they developed into specific cell types. Starting with a single cell type-the H1 human embryonic stem cell - the most widely studied stem cell line to date - the team followed the cells' epigenome from development to different cell states, looking at the dynamics in changes to epigenetic marks from one state to another. Were they methylated, an essential process for normal development, or unmethylated?

What happened to the cells during development? What regulatory processes occurred in the cell lineage?

The scientists found sections of the DNA that activate regulatory genes, which in turn control the activity of other genes, tend to have different amounts of letters of the DNA alphabet, "C" and "G" specifically, depending on when these regulatory genes are turned on during development.

Additionally, regulatory genes that control early development are often located on stretches of DNA called methylation valleys, or DMVs, that are generally CG rich and devoid of epigenetic chemical modifications known as methylation. Consequently, these genes have to be regulated by another epigenetic mechanism, which the authors found were chemical changes called chromatin modifications. Chromatin is the mass of material-DNA and proteins-in a cell's nucleus that helps to control gene expression.

On the other hand, genes active in more mature cells whose tissue type is already determined tend to be CG poor and regulated by DNA methylation. The results suggest that distinct epigenetic mechanisms regulate early and late states of embryonic stem cell differentiation.

"Epigenomic studies of how stem cells differentiate into distinct cell types are a great way to understand early development of animals," says Ecker, who is also a Howard Hughes Medical Institute and Gordon and Betty Moore Foundation Investigator.

"If we understand how these cells' lineages originate, we can understand if something goes right or wrong during differentiation. It's a very basic study, but there are implications for being able to produce good quality cell types for various therapies."

For example, says Matthew Schultz, a graduate student in Ecker's lab, "understanding how development plays out normally could give us clues about how to reverse the process and turn normal adult cells into stem cells to regenerate tissues."

One area where the findings may help is in the study of tumour development. In normal tissue, DMVs are unmethylated, but in cancer, especially breast, colon and lung cancer, they are hypermethylated, suggesting, says Ecker, that alterations in the DNA methylation machinery might be an important mechanism aiding tumour development. He says further investigation is required to develop a greater understanding of this process.

A key type of human brain cell developed in the laboratory grows seamlessly when transplanted into the brains of mice, UC San Francisco researchers have discovered, raising hope that these cells might one day be used to treat people with Parkinson's disease, epilepsy, and possibly even Alzheimer's disease, as well as and complications of spinal cord injury such as chronic pain and spasticity.

This is
Arnold Kriegstein, M.D., Ph.D..

Credit: Susan Merrell/UCSF.

"We think this one type of cell may be useful in treating several types of neurodevelopmental and neurodegenerative disorders in a targeted way," said Arnold Kriegstein, MD, PhD, director of the Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research at UCSF and co-lead author on the paper.

The researchers generated and transplanted a type of human nerve-cell progenitor called the medial ganglionic eminence (MGE) cell, in experiments described in the May 2 edition of Cell Stem Cell. Development of these human MGE cells within the mouse brain mimics what occurs in human development, they said.

Kriegstein sees MGE cells as a potential treatment to better control nerve circuits that become overactive in certain neurological disorders. Unlike other neural stem cells that can form many cell types — and that may potentially be less controllable as a consequence — most MGE cells are restricted to producing a type of cell called an interneuron. Interneurons integrate into the brain and provide controlled inhibition to balance the activity of nerve circuits.

To generate MGE cells in the lab, the researchers reliably directed the differentiation of human pluripotent stem cells — either human embryonic stem cells or induced pluripotent stem cells derived from human skin. These two kinds of stem cells have virtually unlimited potential to become any human cell type. When transplanted into a strain of mice that does not reject human tissue, the human MGE-like cells survived within the rodent forebrain, integrated into the brain by forming connections with rodent nerve cells, and matured into specialized subtypes of interneurons.

These findings may serve as a model to study human diseases in which mature interneurons malfunction, according to Kriegstein. The researchers' methods may also be used to generate vast numbers of human MGE cells in quantities sufficient to launch potential future clinical trials, he said.

Nicholas utilized key growth factors and other molecules to direct the derivation and maturation of the human MGE-like interneurons. He timed the delivery of these factors to shape their developmental path and confirmed their progression along this path. Chen used electrical measurements to carefully study the physiological and firing properties of the interneurons, as well as the formation of synapses between neurons.

Previously, UCSF researchers led by Allan Basbaum, PhD, chair of anatomy at UCSF, have used mouse MGE cell transplantation into the mouse spinal cord to reduce neuropathic pain, a surprising application outside the brain. Kriegstein, Nicholas and colleagues now are exploring the use of human MGE cells in mouse models of neuropathic pain and spasticity, Parkinson's disease and epilepsy.

"The hope is that we can deliver these cells to various places within the nervous system that have been overactive and that they will functionally integrate and provide regulated inhibition," Nicholas said.

The researchers also plan to develop MGE cells from induced pluripotent stem cells derived from skin cells of individuals with autism, epilepsy, schizophrenia and Alzheimer's disease, in order to investigate how the development and function of interneurons might become abnormal — creating a lab-dish model of disease.

One mystery and challenge to both the clinical and pre-clinical study of human MGE cells is that they develop at a slower, human pace, reflecting an "intrinsic clock". In fast-developing mice, the human MGE-like cells still took seven to nine months to form interneuron subtypes that normally are present near birth.

"If we could accelerate the clock in human cells, then that would be very encouraging for various applications," Kriegstein said.

Tuesday, 7 May 2013

A team of New York Stem Cell Foundation (NYSCF) Research Institute scientists report today the generation of patient-specific bone substitutes from skin cells for repair of large bone defects. The study, led by Darja Marolt, PhD, a NYSCF-Helmsley Investigator and Giuseppe Maria de Peppo, PhD, a NYSCF Research Fellow, and published in the Proceedings of the National Academy of Sciences of the USA, represents a major advance in personalized reconstructive treatments for patients with bone defects resulting from disease or trauma.

This advance will facilitate the development of customizable, three-dimensional bone grafts on-demand, matched to fit the exact needs and immune profile of a patient. Taking skin cells, the NYSCF scientists utilized an advanced technique called "reprogramming" to revert adult cells into an embryonic-like state. These induced pluripotent stem (iPS) cells carry the same genetic information as the patient and they can become any of the body's cell types.

The NYSCF team guided these iPS cells to become bone-forming progenitors and seeded the cells onto a scaffold for three-dimensional bone formation. They then placed the constructs into a device called a bioreactor, which provides nutrients, removes waste, and stimulates maturation, mimicking a natural developmental environment.

"Bone is more than a hard mineral composite, it is an active organ that constantly remodels. Blood vessels shuttle important nutrients to healthy cells and remove waste; nerves provide connection to the brain; and, bone marrow cells form new blood and immune cells," said Marolt.

Previous studies have demonstrated the bone-forming potential from other cell sources, yet serious caveats for clinical translation remain. A patient's own bone marrow stem cells can form bone and cartilaginous tissue, not the underlying vasculature and nerve compartments; and, embryonic stem cell derived bone may prompt an immune rejection. The NYSCF scientists chose to work with iPS cells to overcome these limitations, comparing iPS sources with embryonic stem cells and bone marrow derived cells.

"No other research group has published work on creating fully-viable, functional, three-dimensional bone substitutes from human iPS cells. These results bring us closer to achieving our ultimate goal, to develop the most promising treatments for patients," said de Peppo.

While severity varies, bone defects and injuries are currently treated with bone grafts, taken either from another part of the patient's body or a donor bone bank, or with synthetic substitutes. None of these permit complex reconstruction, and they may elicit immune rejection or fail to integrate with surrounding connective tissues. For trauma patients, suffering from shrapnel wounds or vehicular injury, these traditional treatments provide limited functional and cosmetic improvement.

After a comprehensive in vitro analysis of the generated bone, the NYSCF team assessed stability when transplanted in an animal model to address a major concern for iPS-based cell therapies. Undifferentiated iPS cells can form teratomas, a type of tumour. The iPS cell-derived bone substitutes were implanted under the skin of immune compromised mice. After 12 weeks, the explanted constructs matured and showed no malignancies but complete maturation of bone tissue, while blood vessel cells began to integrate along the grafts. These results indicate the stability of the bone substitutes.

The scientists caution that although these results represent a major advance, further research is necessary before skin cell-derived bone grafts reach patients. Next steps include protocol optimization and the successful growth of blood vessels within the bone.

"Following from these findings, we will be able to create tailored bone grafts, on demand, for patients without any immune rejection issues," said Susan L. Solomon, CEO of NYSCF.

"This is not a good approach, it is the best approach to repair devastating damage or defects."

Beyond potential therapeutic relevance, these adaptive bone substitutes may be implemented to model bone development and different pathologies. Analysis could enrich current understanding and identify potential drug targets.

Duke University biomedical engineers have grown three-dimensional human heart muscle that acts just like natural tissue. This advancement could be important in treating heart attack patients or in serving as a platform for testing new heart disease medicines.

The "heart patch" grown in the laboratory from human cells overcomes two major obstacles facing cell-based therapies – the patch conducts electricity at about the same speed as natural heart cells and it "squeezes" appropriately. Earlier attempts to create functional heart patches have largely been unable to overcome those obstacles.

The source cells used by the Duke researchers were human embryonic stem cells. These cells are pluripotent, which means that when given the right chemical and physical signals, they can be coaxed by scientists to become any kind of cell – in this case heart muscle cells, known as cardiomyocytes.

"The structural and functional properties of these 3-D tissue patches surpass all previous reports for engineered human heart muscle," said Nenad Bursac, associate professor of biomedical engineering at Duke's Pratt School of Engineering.

"This is the closest man-made approximation of native human heart tissue to date."

The results of Bursac's research, which is supported by the National Heart Lung and Blood Institute, were published on-line in the journal Biomaterials.

Bursac said this approach does not involve genetic manipulation of cells.

"In past studies, human stem cell-derived cardiomyocytes were not able to both rapidly conduct electrical activity and strongly contract as well as normal cardiomyocytes," Bursac said.

"Through optimization of a three-dimensional environment for cell growth, we were able to 'push' cardiomyocytes to reach unprecedented levels of electrical and mechanical maturation."

The rate of functional maturation is an important element for the patch to become practical. In a developing human embryo, it takes about nine months for a neonatal functioning heart to develop and an additional few years to reach adult levels of function; however, advancing the functional properties of these bioengineered patches took a little more than a month, Bursac said. As technology advances, he said, the time should shorten.

"Currently, it would take us about five to six weeks starting from pluripotent stem cells to grow a highly functional heart patch," Bursac said.

"When someone has a heart attack, a portion of the heart muscle dies," Bursac said.

"Our goal would be to implant a patch of new and functional heart tissue at the site of the injury as rapidly after heart attack as possible. Using a patient's own cells to generate pluripotent stem cells would add further advantage in that there would likely be no immune system reaction, since the cells in the patch would be recognized by the body as self."

In addition to a possible therapy for patients with heart disease, Bursac said that engineered heart tissues could also be used to effectively screen new drugs or therapies.

"Tests or trials of new drugs can be expensive and time-consuming," Bursac said.

"Instead of, or along with testing drugs on animals, the ability to test on actual, functioning human tissue may be more predictive of the drugs' effects and help determine which drugs should go on to further studies."

Some drug tests are conducted on two-dimensional sheets of heart cells, but according to Bursac, the 3-D culture model provides a superior environment for functional maturation of cells. This is expected to better mimic real-world heart muscle responses to different drugs or toxins. Engineered heart tissues made with cells from patients with a cardiac genetic disease could be used as the model to study that disease and explore potential therapies.

The current experiments were conducted on one human pluripotent stem cell line. Bursac and his colleagues have reproduced their findings on two other cell lines and are testing additional lines. They are also planning to move to larger animal models to learn how the patch would become functionally integrated with its host and how the patch establishes connections with the circulatory system.

Monday, 6 May 2013

The human body contains trillions of cells, all derived from a single cell, or zygote, made by the fusion of an egg and a sperm. That single cell contains all the genetic information needed to develop into a human, and passes identical copies of that information to each new cell as it divides into the many diverse types of cells that make up a complex organism like a human being.

If each cell is genetically identical, however, how does it grow to be a skin, blood, nerve, bone or other type of cell? How do stem cells read the same genetic code but divide into very different types?

The
apical tip of fruit fly testis containing germ line

stem cells and differentiating germ cells. Copies of

Y chromosome are marked with either red or blue.

Using this method, the authors discovered that

germ line stem cells inherit specific copies of Y (and

X) chromosomes. Credit: Yukiko
Yamashita.

Researchers at the University of Michigan have found the first direct evidence that cells can distinguish between seemingly identical copies of chromosomes during stem cell division, pointing to the possibility that distinct information on the chromosome copies might underlie the diversification of cell types.

Scientists in the lab of Life Sciences Institute researcher Yukiko Yamashita explained how stem cells can distinguish between two identical copies of chromosomes and distribute them to the daughter cells in a process called non-random chromosome segregation. They also described the genes responsible. Their work is scheduled to be published online May 5 in Nature.

"If we can figure out how and why cells are dividing this way, we might be able to get a glimpse of how we develop into a complete human, starting from a single cell," Yamashita said.

"It is very basic science, but understanding fundamental biological processes always has wide-ranging implications that could be exploited in therapeutics and drug discovery."

During the cell division cycle, the mother cell duplicates its chromosomes, generating two identical sets. When the cell divides to become two cells, each cell inherits one set of chromosome copies. In many divisions, the daughter cells are identical to the mother — one skin cell becomes two, for instance.

But in a process called asymmetric division, a cell divides into two daughters that are not identical — a skin stem cell divides into another skin stem cell and a regular skin cell, for example. In that case, the genetic information within the chromosome copies remains the same, but the type of cell, or "cell fate," is different.

The Yamashita lab used stem cells from the testes of the fruit fly Drosophila to study the process of cell division.

"The Drosophila germ line stem cell can be identified at a single-cell resolution, so they are an ideal model," Yamashita said.

The stem cells cluster and are easy to identify; they divide to produce another germ line stem cell and a differentiating cell called a gonialblast, which goes on to eventually become a sperm cell.

The researchers marked the copies of each chromosome in the Drosophila stem cells as they divided. Using this method, they tracked the tendency of the X and the Y chromosome copies to move to the daughter germ line stem cell or to the gonialblast. They were able to demonstrate that copies of X and Y chromosomes (but not other chromosomes) are distinguished and delivered to the daughter cells with a striking bias.

This is the first direct evidence that cells indeed have an ability to distinguish identical copies of chromosomes and separate them in a regulated manner. This ability has been suspected and hypothesized, but never proven.

"We think maybe specific epigenetic information is transmitted to the germ line stem cell and to the gonialblast."

The findings suggest that the information on the X and Y chromosomes that makes this division possible is primed during gametogenesis — the process of creating ovum or sperm cells — in the parents.

Many other cells throughout the body are able to divide into two different types, especially during embryonic development. Yamashita's next steps are to explore whether the non-random chromosome segregation seen in Drosophila is a widespread phenomenon that is shared by mammals, including humans.

A University of Wisconsin-Madison research group has converted skin cells from people and monkeys into a cell that can form a wide variety of nervous-system cells — without passing through the do-it-all stage called the induced pluripotent stem cell, or iPSC.

Standing
at centre, Su-Chun Zhang, professor of

neuroscience
in the School of Medicine and Public

Health.,
talks with his staff as they prepare stem-cell

cultures
in the Zhang's research lab at the Waisman

Center
at the University of Wisconsin-Madison on

March
8, 2013. Pictured at right are postdoctoral

students
Yan Liu, background, and Lin Yao,

foreground.
Credit: Photo by Jeff Miller/UW-Madison.

Bypassing the ultra-flexible iPSC stage was a key advantage, says senior author Su-Chun Zhang, a professor of neuroscience and neurology.

"iPSC cells can generate any cell type, which could be a problem for cell-based therapy to repair damage due to disease or injury in the nervous system."

In particular, the absence of iPSC cells rules out the formation of tumours by pluripotent cells in the recipient, a major concern involving stem cell therapy.

A second advance comes from the virus that delivers genes to reprogram the adult skin cells into a different and more flexible form. Unlike other viruses used for this process, the Sendai virus does not become part of the cell's genes.

Jianfeng Lu, Zhang's postdoctoral research associate at the UW–Madison Waisman Center, removed skin cells from monkeys and people, and exposed them to Sendai virus for 24 hours. Lu then warmed the culture dish to kill the virus without harming the transforming cells. Thirteen days later, Lu was able to harvest a stem cell called an induced neural progenitor. After the progenitor was implanted into new-born mice, neural cells seemed to grow normally, without forming obvious defects or tumours, Zhang says.

Other researchers have bypassed the pluripotent stem cell stage while turning skin cells into neurons and other specialized cells, Zhang acknowledges, but the new research, just published in Cell Reports, had a different goal.

"Our idea was to turn skin cells to neural progenitors, cells that can produce cells relating to the neural tissue. These progenitors can be propagated in large numbers."

The research overcomes limitations of previous efforts, Zhang says. First, the Sendai virus, a kind of cold virus, is considered safe because it does not enter the cell's DNA, and it is killed by heat within 24 hours. (This is quite similar to the fever that raises our temperature to remove cold virus.) Second, the neural progenitors have a greater ability to grow daughter cells for research or therapy. Third, the progenitor cells are already well along the path toward specialization, and cannot become, say, liver or muscle cells after implantation. Finally, the progenitors can produce many more specialized cells.

The neurons that grew from the progenitor had the markings of neurons found in the rear of the brain, and that specialization can also be helpful.

"For therapeutic use, it is essential to use specific types of neural progenitors," says Zhang.

Progenitor cells grown from the skin of ALS (Lou Gehrig's disease) or spinal muscular atrophy patients can be transformed into various neural cells to model each disease and allow rapid drug screening, Zhang adds.

Eventually, the process could produce cells used to treat conditions like spinal cord injury and ALS.

"These transplantation experiments confirmed that the reprogrammed cells indeed belong to cells of the intended brain regions and the progenitors produced the three major classes of neural cells: neurons, astrocytes and oligodendrocytes," Zhang says.